Thermal ink-jet printers are fast and durable and are known to produce high definition color printouts easily and cost effectively. A typical thermal ink-jet printer is disclosed in U.S. Pat. No. 4,728,963, assigned to Hewlett-Packard Company, Palo Alto, Calif.
The '963 patent discloses an inkjet printer employing a plurality of resister elements to expel droplets of ink to a plurality of nozzles. Upon energizing a particular resister element, a droplet of ink is expelled through a process of vaporization through the nozzle toward the print medium, such as paper, fabric or plastic film. The firing of ink droplets can be microprocessor controlled. As the ink cartridge containing the nozzles is moved repeatedly across the width of the medium to be printed, each of the nozzles is caused to eject ink or refrain from ejecting ink according to the instructions provided by the microprocessor. In order to obtain multi-colored printed images, a plurality of ink-jet cartridges are used, each having a chamber holding a different color ink.
Ink-jet printers are known to have two common deficiencies associated with ink saturation into a porous media, such a paper. The first is that the often liquid inks absorb into the cellulosic fibers of the paper, causing them to swell. This generates unacceptably wavy or "cockled" formations in the ordinarily flat paper media. "Paper cockle" can cause a degradation of print quality due to uncontrolled spacing between the pen and paper, and can sometimes cause the paper to come in contact with the print head, which results in a low quality images and print. The second problem is that adjacent colors of colored ink-jet print tend to run or bleed into one another, causing a reduction in resolution and clarity of the final image.
Heating elements have been proposed for drying the ink rapidly after it has been printed. This has helped to reduce smearing, but has only limited effectiveness in reducing paper cockle or ink migration that occurs during printing, and in the first few fractions of a second after printing.
One solution provided by U.S. Pat. Nos. 5,406,321 and 5,633,668, also assigned to Hewlett-Packard Company, and hereby incorporated by reference, includes a preconditioning preheater disposed along the medium path for heating the medium before it reaches the print area. The preheater of the '321 patent includes a thin heating surface, such as a thin flexible film having a large area suspended in air by a support structure. The preheater has very low thermal mass so as to avoid long warmup intervals, and is supported by securing one edge of the preheater film along the print area with spring tensioners for keeping the film taut. The film is designed to assume a curved edge support, while suspending most of the area of the preheater in air.
While thin film heating elements are known to effectively precondition paper for ink-jet printers, such members represent an additional component of a thermal ink-jet heater, adding to its overall cost. Moreover, thin film heating elements can very easily be dislodged or damaged during heavy use, and may unintentionally cause temporary or permanent thermal damage to the often polymeric medium path immediately adjacent to the print area.
Accordingly, there remains a need for a heating element for preconditioning print media which is more cost effective, and which does not present a possible obstacle in the media pathway or cause thermal distortion of polymeric printer components during use.